When creating a backup, check if public/uploads exists.

If there is not a `public/uploads` directory, backup fails with:

```
Dumping uploads ...
rake aborted!
No such file or directory - /home/git/gitlab/public/uploads
```

This behavior was reported in the [forum](https://groups.google.com/forum/?fromgroups=#!topic/gitlabhq/So4mYIRFsX8).